A Novel Passive Interface Circuit With Gain for Nongenerating Sensors in Differential or Bridge Configuration

Abstract

A novel passive interface circuit with gain for nongenerating (active, i.e., excited/driven) sensors in differential or bridge configuration is presented. The circuit is very simple and consists of only one transformer in the output diagonal of the equivalent bridge configuration comprised of sensor(s), bridge completion impedances, and excitation source. It can be used with resistive, capacitive, inductive, eddy-current, and similar sensors for measurement of temperature, displacement, pressure, torque, etc. The gain achieved depends on the transformer voltage ratio, $n$ . Unlike the usual transformer application for voltage raise (gain) where the gain is proportional to $n$ , in this circuit, the high gain is achieved with $n$ close to 1. This is due to positive feedback introduced into the circuit. Transformers with voltage ratios close to 1 exhibit much less leakage and smaller losses which can degrade the (ideal) gain. With current on-chip transformers or miniature transformers available on the market, the practical gain can reach 20 dB. The circuit can be easily integrated into various MEMS sensors. As the circuit is entirely passive, its application extends far beyond the usual semiconductor temperature limits. Depending on the transformer technology, the circuit can be used in a very cold (e.g., cryogenic) or very hot environment (e.g., steam and gas turbines, jet engines, rockets, down-hole drilling, etc.) or exposed to nuclear radiation.